In order to increase the sensitivity of surface plasmon resonance sensors through planar metallic film closely coupled to nano- gratings and use grating period to adjust the wavelength of resonance reflection,sub-micron gratings is needed. The fabrication technique for sub-micro gratings structure is introduced. The sub-micron gratings are fabricated by synchrotron radiation (SR) lithography with SR source at Ritsumeikan University,Japan,and polymethylmethacrylate (PMMA) is used as X-ray resist. The 250 nm-width with 500 nm period pattern is successfully fabricated. The sub-micron grating with the aspect ratio of 8 is achieved. The lithography parameters such as proximity gap of exposure,the exposure dosage,and the development time influencing the structure are optimized to fabricate the gratings.